The present invention is directed to a method for producing a superconductor layer of YBa.sub.2 Cu.sub.3 O.sub.7 on a substrate composed of sapphire.
Superconductor layers of high-quality YBa.sub.2 Cu.sub.3 O.sub.7 on substrates composed of sapphire are required, for example, in the manufacture of microwave devices. In the manufacture of such a superconducting layer of YBa.sub.2 Cu.sub.3 O.sub.7 on a substrate composed of sapphire by a direct deposit of the superconducting layer on the substrate, there is the risk of the diffusion of the aluminum from the sapphire through the superconducting layer. The temperature of the substrate must, therefore, be kept so low during the deposition that this diffusion fails to occur.
The deposition at low temperatures of the substrate, however, has the disadvantage that the superconducting layer has a faulty crystal perfection. The crystal perfection is too poor in order to supply good superconducting characteristics, such as transition temperature T.sub.c and critical current density J.sub.c.
It is known, for example see an article by K. Char et al, Appl. Phys. Lett., Vol. 57, 1990, pp. 409-411, to resolve the diffusion problem in that an intermediate layer is applied onto the substrate under the superconducting layer. The intermediate layer must, thereby, grow epitaxially on the substrate, which is composed of a sapphire. Over and above this, the epitaxial growth of the superconducting layer onto the intermediate layer must be possible. With respect to its grating constant, therefore, the intermediate layer must be adapted, first, to the lattice structure of the sapphire substrate and, secondly, to the lattice structure of the superconducting layer of YBa.sub.2 Cu.sub.3 O.sub.7.
In the above-mentioned article by K. Char. et al, Appl. Phys. Lett, Vol. 57, 1990, pp. 409-411, the authors investigated an intermediate layer of SrTiO.sub.3 on a substrate composed of a sapphire under a superconducting layer of YBa.sub.2 Cu.sub.3 O.sub.7. A critical current density of 2.times.10.sup.6 A/cm.sup.2 at 74.degree. K. was, thereby, achieved.